Aerial vehicles, which can include manned airplanes, rotary type unmanned aerial vehicles (UAV) including helicopters, quadcopters, hexacopters, octocopters, and/or fixed wing UAVs can be used to obtain aerial photographs. However, when sensing remote bare soil, the BRDF values can be noticeable, particularly when sensing from low elevations from UAVs or airplanes.
BRDF defines how light is reflected at an opaque surface and can be calculated by taking an incoming light direction (ωi), and outgoing direction (ωr) (taken in a coordinate system where the surface normal (n) lies along the z-axis, and returns the ratio of reflected radiance exiting along the outgoing direction (ωr) to the irradiance incident on the surface from the incoming light direction (ωi)). Each direction ω is itself parameterized by azimuth angle ϕ and zenith angle θ, therefore the BRDF as a whole is a function of 4 variables. The BRDF has units sr−1, with steradians (sr) being a unit of solid angle.
If the BRDF is not calibrated correctly, the sensed soil information may not be accurate due to the roughness of the soil, tillage method (plowing or cultivation causes ridges in the soil), field terrain, and soil mineral content and texture (sand, loam, clay, or silt). In addition, the same effect can be seen for remote vegetation sensing, depending on crop type, how much soil is showing between the plants, and lighting conditions.
BRDF can also cause remote sensing inaccuracies if not corrected. For example, methods that measure BRDF using measurement of lighted surfaces may not be effective when sensing at UAV or manned airplane elevations because lighting sources powerful enough to light up the ground would draw excessive power from the aerial vehicle or could be a danger to people or animals on the ground.
Known systems use rotating mirrors, multiple sensors and/or measurement of lighted surfaces for BRDF calibration. These methods, however, are not ideal for UAVs since they can add weight, are costly, or have mechanical systems that can fail during operation.
Known systems also describe flight patterns, for example, flying in a circular pattern several times for BRDF calculation. However, this is also not ideal since it performs BRDF calibration for one field location and does not correct BRDF for other parts of the field.